Titration of Cu(I) Sites in Cu-ZSM-5 by Volumetric CO Adsorption

Cu-exchanged zeolites are widely studied materials because of their importance in industrial energetic and environmental processes. Cu redox speciation lies at the center of many of these processes but is experimentally difficult to investigate in a quantitative manner with regular laboratory equipment. This work presents a novel technique for this purpose that exploits the selective adsorption of CO over accessible Cu(I) sites to quantify them. In particular, isothermal volumetric adsorption measurements are performed at 50 °C on a series of opportunely pre-reduced Cu-ZSM-5 to assess the relative fraction of Cu(I); the setup is fairly simple and only requires a regular volumetric adsorption apparatus to perform the actual measurement. Repeatability tests are carried out on the measurement and activation protocols to assess the precision of the technique, and the relative standard deviation (RSD) obtained is less than 5%. Based on the results obtained for these materials, the same CO adsorption protocol is studied for the sample using infrared spectroscopy, and a good correlation is found between the results of the volumetric measurements and the absorbance of the peak assigned to the Cu(I)–CO adducts. A linear model is built for this correlation, and the molar attenuation coefficient is obtained, allowing for spectrophotometric quantification. The good sensitivity of the spectrophotometric approach and the precision and simplicity of the volumetric approach form a complementary set of tools to quantitatively study Cu redox speciation in these materials at the laboratory scale, allowing for a wide range of Cu compositions to be accurately investigated.


Details on the volumetric apparatus
The cell that has been employed for volumetric measurements is shown in Figure S1. It is composed of a quartz burette that contains the sample, linked to a valve that can be connected to a vacuum line enabling thermochemical treatment and exposure to different reactants in the gas phase. The sample environment can be inserted in a thermal jacket that can be filled with a thermostatic fluid, allowing for precise isothermal conditions. Figure S1. Picture of the cell employed for isothermal volumetric adsorption measurements. Italian patent application n°102020000005014 filed on March 9 th , 2020 and PCT n. PCT/IB2021/051769 filed on March 3 rd , 2021.

XANES spectra of oxidized and reduced (0.45)Cu-MFI(11.5) material
Cu K-edge XAS spectra were collected on the (0.45)Cu-MFI(11.5) material, selected as representative sample, at the BM31 beamline at ESRF (Grenoble), in transmission mode using a Si(111) double-crystal monochromator. The incident (I0) and transmitted (I1) X-ray intensities were detected in the range within 8780 and 10500 eV (40 min/scan, 3 spectra collected per sample) using two ionization chambers filled with a mixture of He and Ar. Finally, a third ionization chamber (I2) was used for the simultaneous collection of a Cu metal foil XANES spectrum employed in the energy calibration procedure. The collected XAS spectra were then aligned in energy and normalized to unity edge jump using the Athena software from the Demeter package 1 . The samples were prepared for XAS measurements in the form of self-supporting pellets, with thickness optimized for transmission measurements. The pellets were treated as for the IR/volumetric measurement, also interrupting the treatment after the pre-oxidation of the material in order to collect a reference spectrum of a deeply oxidized state. After treatment, pellets have been transferred in a N2-filled glovebox (O2 and H2O concentrations < 0.5 ppm) and sealed in polyethylene bags in order to avoid the reexposure to the external environment. The measurements have been performed with the pellets still inside the plastic envelope, guaranteeing the desired chemical state is maintained. Figure S2 depicts the XANES spectra of the (0.45)Cu-MFI(11.5) material after exposure to O2 and NH3 at 500°C. As can be seen from the spectral profiles, the two states are characteristic of deeply oxidized (Cu(II)) and reduced (Cu(I)) states respectively, as already reported in the literature for these materials 2 . The absence of an evident 1s→3d transition (8977.5 eV) in the spectrum of the material reduced in NH3 at 500 °C suggests the residual fraction of Cu(II) is certainly lower than 10%. The sharp 1s→4p transition (8983 eV) is instead confirming the existence of low-coordinated Cu(I) sites, compatibly with a prevalence of "naked" Cu(I) ions directly interacting with the zeolite framework. In the case of a major contribution of (quasi-)linear Cu(I)(NH3)x (with x = 1 or 2) adducts, a higher intensity for the 1s→4p transition, i.e. comparable with that of the white line peak, would have been expected 3 .   Figure S3 shows the IR spectrum of the (0.48)Cu-MFI(25) sample after activation and upon interaction with CO in the full acquisition range. Traces of adsorbed NH3 are still present on the sample even after long evacuation at 550°C, as can be seen from the IR spectra of the materials after evacuation. This is suggested by the presence of characteristic bands associated with NH3 at 3370 and 3280 cm -1 (Figure S4a) 4 . The residual NH3 on the material is most probably ligating part of the Cu in the sample; this can be inferred by the IR spectra of the material in the low frequency zone upon interaction with CO, as exemplified for (0.48)Cu-MFI(25) in Figure S4b. The band at 2133 cm -1 only forms in a relevant amount for the (0.48)Cu-MFI(25) sample ( Figure S5); this is probably due to the very scarce amount of Brønsted acid sites present in this material compared to the others. The relatively high amount of Cu in this sample could retain an amount of NH3 that cannot migrate to nearby proton sites, effectively shifting the equilibrium towards a NH3/CO mixed ligand complex for a higher fraction of the Cu sites. Figure S6 shows the complete dataset of isotherms collected for the five materials characterized in this paper. Figure S6. CO adsorption isotherms performed at 50°C on the pre-reduced Cu-MFI samples. Orange: primary isotherm (P). Brown: secondary isotherm (S). Red: difference between the primary and secondary isotherm (P-S), used to calculate the amount of irreversibly bound CO. Figure S7. Graphical  Cu environment (in terms of distance to neighbouring O and C atoms) changes upon adsorption with a relative trend that follows what has been previously reported in the literature for similar materials 5

Normalization of the IR spectra of the materials interacting with CO
The spectrum of each material after interaction with CO and outgassing has been arbitrarily shifted to 0 in correspondence with the valley at 1745 cm -1 ; this point was chosen as the best compromise between minimal interference from scattering effects and minimum absorption from the sample or other molecules. The spectra were then opportunely scaled to a value of absorbance 0.33647 (i.e., the value for one of the samples) at the maximum of the framework modes combination band at 1880 cm -1 . This normalization should account for differences in the thickness of the pellets used for the four materials, which were prepared with the intention of keeping the monocarbonyl band in the 0.3-1.0 range for absorbance. Finally, the corresponding spectrum of the activated sample prior to exposure to CO was subtracted to each of them to take scattering effects into account. Integration was performed on the normalized spectra in the 2180-2120 cm -1 range. S10

Details on the statistical analysis for the spectrochemical linear model
As mentioned in the main text, the integrated molar attenuation coefficient for the Cu(I)monocarbonyl stretching peak has been calculated by determining the slope of the equation: This equation is derived from the Beer-Lambert law, and in this form it does not present an intercept. General linear least-squares regression normally estimates both a slope and an intercept in the fitting procedure, as shown in Figure S9. A simple Student's t-test, however, suggests that the intercept of this model is statistically equivalent to 0; for this reason, the model presented in Figure 4 is the one that has been used to calculate the slope. This methodology is the one suggested by the NIST handbook for linear calibration 6 . Finally, Figure S10 reports the Residual Plot for the model shown in Figure 4. Figure S10. Residual plot for the model presented in Figure 4 (main text).
As can be noticed, the residuals are distributed in a somewhat random manner, and their mean absolute value does not appear to be increasing or decreasing with the independent variable. Thus, the homoscedasticity of the model, together with a good Pearson correlation coefficient (R 2 =0.9953), confirms the overall robustness of the model in the explored range for these samples.